As glazing substitutes, shatterproof or highly shatter resistant transparent materials have been widely utilized for these decades. For example, plastic substrates, especially polycarbonate resins have superior transparency, impact resistance and heat resistance and are currently used as structural members instead of glass in a variety of applications including building and vehicle windows and instrument covers.
The polycarbonate resins, however, are inferior to glass in surface properties such as mar resistance and weather resistance. It is desired to improve the surface properties of polycarbonate resin parts. Nowadays, polycarbonate resin parts for use as vehicle windows and acoustic barrier walls along highways are required to withstand more than 10 years of weathering.
Known means for improving the weather resistance of polycarbonate resin parts include the lamination of a weather resistant acrylic resin film on the surface of a polycarbonate resin substrate and the formation of a ultraviolet absorber-containing resin layer on the resin surface, for example, by co-extrusion.
For improving the mar resistance of polycarbonate resin parts, it is known to coat thermosetting resins such as polyorganosiloxanes and melamine resins and to coat photo-curable resins such as polyfunctional acrylic resins.
As to the manufacture of transparent articles having both weather resistance and mar resistance, JP-A 56-92059 and JP-A 1-149878 disclose ultraviolet-absorbing transparent substrates having a primer layer loaded with a large amount of UV absorber and a protective coating of colloidal silica-laden polysiloxane paint overlying the primer layer.
However, several problems arise with this approach. The addition of a large amount of UV absorber to the primer layer can adversely affect the adhesion of the primer layer to the substrate or the overlying protective coating of colloidal silica-laden polysiloxane. During heat curing step, the UV absorber can volatilize off. On outdoor use over a long period of time, the UV absorber will gradually bleed out, causing undesired impacts like cracking, whitening or yellowing. From the mar resistance standpoint, it is impossible to add a large amount of UV absorber to the protective coating of colloidal silica-laden polysiloxane.
It is also known from JP-A 8-151415 that a mixture of a benzotriazole-derived UV absorbing vinyl monomer or a benzophenone-derived UV absorbing vinyl monomer and another vinyl monomer copolymerizable therewith is used in coating compositions, which are effective for forming protective coatings on surfaces of synthetic resins. Since these protective coatings are based on vinyl polymers, their mar resistance is limited.
JP-A 2001-114841, Japanese Patent No. 3,102,696, and JP-A 2001-214122 disclose coating compositions comprising copolymers of a benzotriazole-derived UV absorbing vinyl monomer or a benzophenone-derived UV absorbing vinyl monomer, an alkoxysilyl group-containing vinyl monomer, and another vinyl monomer copolymerizable therewith. Allegedly they form weather resistant coatings on resin substrates in tight adhesion for yielding multilayer-coated resin articles.
JP-A 2000-114841 lacks examples of using a copolymer coating composition as the undercoat. Because of the vinyl polymer, the coating of this composition alone is limited in mar resistance improvement.
In Japanese Patent No. 3,102,696, a coated article endowed with mar resistance and weather resistance is obtained by using a copolymer coating composition as a primer and forming a colloidal silica-laden polysiloxane resin coating thereon. The weather resistant coating suppresses yellowing. However, the primer layer can become extremely hard due to the crosslinked network of alkoxysilyl groups, or undergo post-crosslinking of residual alkoxysilyl or hydroxysilyl groups over time. Thus the coating is likely to strain, allowing frequent failures like cracks and stripping. Long-term weather resistance is still insufficient.
Also in JP-A 2001-214122, an article is coated with a colloidal silica-laden polysiloxane resin coating subsequent to a copolymer coating composition as a primer. The weather resistant coating suppresses yellowing. However, the coating fails to follow environmental temperature changes, especially changes at relatively high temperature, allowing frequent failures like cracks and stripping.